Polycarbonate resin may often contain certain impurities which in turn affect their property performance in the final molded article. For example, sulfate, chloride and fluoride ions, if present in sufficient quantities, will adversely affect color and processability of polycarbonate resins for producing pre-chip silicon wafer carriers, or computer hard drive platter carriers. Specifically, the sulfate ions can react with residual ammonia on the surface of the silicon wafer to form ammonium sulfate which forms a white residue on the surface of a silicon pre-chip wafer. The wafer then requires cleaning before being processed into a computer chip. Additional heat phase exposure such as injection molding, extrusion or compounding thereof, may also induce discoloration of the polycarbonate resin. Even with phosphite stabilizers, yellowing of the polycarbonate and hydrolysis of the phosphite can occur, particularly at high processing temperatures. It is believed that the phosphite stabilizers, which are susceptible to hydrolysis at elevated processing temperatures, form in-situ acid species which can then react with the polycarbonate. Such reactions could possibly increase chain scission and give rise to side-reactions which can eventually generate color in the molded article. This undesirable coloration can also occur during extrusion, compounding or molding of the polycarbonate resin. Moreover, polycarbonate/PTFE resin parts in hard drives may contain quantities of fluoride ions that may cause corrosion of the hard drive platter.
In order to achieve certain desired or improved properties, additives may be introduced into polycarbonate resin compositions during extrusion, compounding or injection molding. For example, small amounts of polytetrafluoroethylene (PTFE) may be added to achieve a polycarbonate/PTFE blend having a lower coefficient of friction than polycarbonate alone.
Additionally, it is known to stabilize polycarbonate resin against discoloration by adding phosphites and/or epoxies which act as stabilizers. The stabilizers are thought to neutralize impurities which would otherwise cause coloration of the polycarbonate compositions. Such additives are broadly disclosed in such U.S. Pat. Nos. as 4,381,358, 4,358,563 and 3,673,146. Unfortunately, these stabilizers may adversely affect processability.
Accordingly, if certain known impurities could be removed from polycarbonate compositions without using such additives to neutralize the impurities, the resultant composition would avoid both the processability problems caused by additives and problems caused by impurities (e.g., coloration).
Therefore, it is an object of this invention to provide a process for reducing ionic impurities in a polycarbonate resin.
Another object of this invention is to reduce ionic impurities in a polycarbonate resin during melt blending of the polycarbonate composition.
Yet another object of the invention is to reduce sulfate, chloride and fluoride ions in a polycarbonate resin.
Still another object of this invention is to produce a polycarbonate resin having reduced ionic impurities.